Process for producing crystalline silicon over a substrate and removal therefrom



April 7, 1964 K. E. BEAN ETAL.

y PRocEss FOR UCING CRYSTALLINE SILICON EFROM PROD Filed Dec. 19, 1958OVER A SUBSTRATE AND REMOVAL THER United States Patent O PRCESS FRPRODUCING CRYSTALLEIE SlLl- CGN OVER A SUBSTRATE AND REMOVAL Medcalf,Miami, Okla., Cincinnati,

This invention relates to a method for producing crystalline silicon ofa type adapted to be directly introduced into zone refining equipment.More particularly, it relates to a method for obtaining large pieces ofcrystalline silicon by deposition on a hot filament comprised of a metalother than silicon in such a manner that the silicon is readilyremovable in integral piece form from the fiament on which it has beendeposited.

Semiconductor devices, which today are a primary outlet for elementalsilicon, require silicon of the highest purity. Such standards of purityare best met, it has been found, by final purification of the semi-pureproduct with zone refining methods. In the recent past, semi-purecrystalline silicon in the bar form adapted for the utilization of zonerefining purification methods has occasionally been produced by vaporphase deposition on a slender, elongated filament, or core, which itselfis made of silicon, so as to act as a seed crystal and so as to becomean integral part of the deposit thereon. Silicon filaments also havebeen obtained by pulling out a filament from a molten mass of the metal;this method is inherently both tedious and expensive. While both methodshave the advantage that the bar subseqently deposited on the filament isrelatively free of core impurities, and each provides a bar or rod shapewhich is adapted for direct introduction into refining equipment of thefloating zone type, the use of silicon as a filament upon whichdeposition of silicon is effected is very expensive as to initial and tooperating costs and the technique has not found favor despite itstheoretical advantages.

Alternatively, crystalline silicon has been deposited on filaments ofmetals other Such filaments are often three or four feet in length andof small diameter. When the deposition on the filament is completed, thesilicon must be cracked away from the bar before final purification ofthe metal may begin. The diiculty characteristic of this method is thatone cannot simply remove the filament or core by pulling itlongitudinally out of the silicon sheathing, because the filamentinevitably breaks under the strain. The difculty in removal of thefilament stems not from an especial affinity between the silicon and thefilament material but rather from the fact that microscopicprotuberances over the surface of the filament project into thesuperimposed layer of metal deposited thereon so as to prevent slidingmovement necessary to disengage the one from the other by pull alone.For this reason, it has been necessary either to break the deposit awayfrom the filament in small pieces which at the same time destroy or maydestroy the filament, or to latch the bar and filament for anexcessively long time, perhaps several weeks, in a strong acidic bathof, for example, hydrofluoric acid. In the practice of the formermethod, in which the bar is broken up to facilitate removal of thefilament, the small pieces f silicon must be remelted and cast into barform before final purification may begin, while in the latter method,complete removal of the filament is not always effected, and even if itis, the long leaching process causes the produce to become contaminatedwith acidic solution. Because of these concomitant difficulties, neitherof these methods has been entirely suitable. Due to the increasingdemand for silicon of high purity for use in semi-conthan silicon, suchas tantalum.`

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ductor and other devices, and the increasing application of the fioatingzone purification process in industry, it is evident that a method ofproduction for silicon is highly desirable in which the silicon, afterproduction, is adapted to be directly introduced into a floating zone,without prior melting, cutting or leaching.

A principal objective of the present invention has been to provide amethod for deposition of silicon crystal bars on metallic filamentswhereby the bars may be removed intact from the filament withoutbecoming contaminated to an appreciable extent in the process. Furtherobjectives have been to provide a method which coincidentally preservesthe filament, so that it is available for reuse, and which eliminatesthe need for time-consuming leaching procedures.

We have discovered that crystalline silicon of bar shape shape which isreadily removable from the filament on which it is deposited may beproduced by first coating the filament as a step of the reductionprocess itself with a layer of silicon of very fine particle structureor of amorphous structure, and subsequently depositing the crystallinesilicon on top of the powdery substrate so formed.

Otherwise expressed, we have discovered that silicon may be caused todeposit upon a hot filament in an amorphous or finely divided particleform as well as in the form of a solid crystalline material, and thatdeposition of either of such forms selectively may be caused to occur bycontrol of the operating conditions under which the deposition iseffected. More specifically, we have discovered that if a layer orsubstrate of amorphous silicon is caused first to be deposited upon thefilament, after` which the silicon is caused to be deposited in thesolid or continuous metallic state, then the latter deposit may bestripped longitudinally from the filament as an intact tube or hollowbillet by reason of the mobility which is afforded by the interveninglayer of amorphous silicon. Where the filament is eighteen inches to twofeet or so in length, which is a length suitable for practicaloperations, the deposited billet of crystalline silicon readily may bepulled manually from the filament, thereby providing a billet which isphysically intact and which, as a single piece, may be charged directlyinto a fioating zone unit or other zone refining equipment either aloneor with other pieces.

The amorphous silicon, constituting the interlayer between thecrystalline or solid state silicon and the filament, apparentlyfacilitates removal of the latter `from the former by reason of itschalk-like or powdery property. It is believed that the discreteparticles, though generally adherent to the filament, either are, orreadily become, mobile if a longitudinal pull is exerted on the filamentrelative to the layer of pure crystalline material. This mobility of the4amorphous layer may occur as a rolling motion of the particles betweenthe filament and Ithe solid metallic layer of silicon surrounding it, orit may be that the amorphous silicon coating simply constitutes a layerwhich prevents microscopic protucerances on the surface of 4the filamentfrom forming a locking engagement with the crystalline silicon depositover the layer and which is easily ruptured by reason of its amorphousnature or by reason of the fine particles of which it is composed. Theamorphous layer need only be very thin to accomplish lthe desiredresult. The crystalline deposit need only be thick enough to be capableof withstanding the longitudinal pull to separate it from the filament,though i in the preferred practice of the invention, the deposition ofcrystalline silicon is continued until a billet of substantial thicknessis built up thereby to increase the productivity of the hot wireequipment. Upon separation from the billet, the filament is unharmed andmay be re-used repeatedly.

The 'deposition of silicon `upon a hot wire as produced through thermaldecomposition of a 4Vaporous silicon-containing compound, such as`silicon tetrachloride in the presence of a suitable gas atmosphere, iswell-known in the art. In the practice of the present invention, silicondeposition in either the amorphous state or continuous crystalline stategenerally is governed by operating conditions, and the production anddeposition of amorphous silicon is facilitated when a silane such as thetrichlorosilane (SiHCl3) is employed as the vapor source of silicon. Ingeneral, at lower filament temperatures (which of course must be highenough to effect thermal decomposition), formation of the silicon in theamorphous state is favored, while higher filament temperatures favor theformation of a continuous crystalline layer or deposit. -By way ofexplaining the process in this respect, consider, Ifor example, thebehavior of a typical halogen-containing silane, trichlorosilane,SiHCl3. This compound is a liquid at room temperature but boils at 33 C.It has the property of undergoing thermal dissociation at relatively lowtemperatures and atmospheric pressure. In the dissociation, whatapparently occurs is a molecular rearrangement in which the hydrogenatom acts as a reducing agent:

Cl Cl The (SiCl2) is electronically incomplete, silicon normal' In thepreferred method of practicing this invention the filament is made oftantalum. The reason lfor this preference is that because of theintimate contact of the filament and silicon, some of the material ofwhich the filament is made generally is transferred to the crystal as animpurity, and tantalum, having a distribution coe-fficient of 1x10-S,can be removed from the crystal by the floating zone technique morereadily than can other metallic elements. Otherwise expressed, tantalumis used because silicon can so easily be cleansed of it. However, thepractice of the invention is not limited to the use of tantalumfilaments and the use of other metallic elements such as molybdenum,tungsten, and similar metals which are non-reactive with the gaseoussilicon source are intended to be included within the scope of thisimprovement in the art.

The precise extent, if any, to which the filament metal itselfchemically enters into the chemical reduction of the silane is notknown. Tantalum is inert or at least relatively unreactive to drygaseous silanes and on that basis it would not be expected to act otherthan asa heat source and as the core on which the deposit forms. It maybe, however, that the initial coating which forms on the tantalumfilament is not elemental silicon but rather is tantalum silicide,resulting from the direct union of the two elements:

Ta-i-Si-TaSi which in `some manner is then reduced. The process takesplace in the presence of hydrogen and helium gas, as will subsequentlybe explained, and the hydrogen may be the reducing agent. In any event,the substrate protects the filament from being eroded by the hydrogen,which otherwise might attack it by formation of the hydride, TaHil.

It is also to be noted that while the substrate is referred to hereinand previously has been identified as amorphous silicon, it may be amixture of substances rather than a pure compound, and some crystallinesilicate may indeed be present in this layer. On that account when,hereinafter, the substrate is referred to as amorphous silicon, it willbe understood that this is intended as a generic term used incontradistinction to the term crystalline silicon andencompasses bothline particle silicon alone and a mixture of fine particle and amorphoussilicon.

The substrate may be visually recognized, provided the filament is notred hot, by its yellowish, greenish or reddish color as contrasted withythe characteristic silvery color of crystalline silicon. `(Of course,if the filament is energized, the entire substrate glows red at theoperating tempera-ture.) Because of the characteristic difference incolor, simply by permitting the filament to cool during the depositionprocess one may know whether conditions inside the reactor are promotingthe formation of crystalline or of amorphous silicon.

The use of trichlorosilane as the silicon supplier in the reaction ispreferred because `of the ease with which amorphous silicon may bedeposited from it, and because it is a readily' available source ofsilicon which is amenable to hot Wire deposition, but those `skilled inthe art readily will understand that `other suitable :silanes may beemployed as silicon source materials in .accordance with the teachingsof this invention.

The following detailed description of the invention is best understoodin reference to the accompanying drawing in Iwhich FIGURE 1 is aschematic diagram 4of typical apparatus adapted for the practice of theinvention.

The arrangement shown in FIGURE 1 essentially comprises a reactor 1 anda boiler 2r. The reactor is an elongated tubular structure having sealedtop and bottom ends. A slender filament 31` of tantalum extends` betweenthe two ends, so that it is essentially parallel to the axis of thereactor. Externally, the two ends yof the filament are connected to aconventional source of power whereby the filament may be heated throughits internal electrical resistance toa tempenature of about 1100 C. Thepower source shown is a generator 4, connected to the filament through aswitch 5 and a rheostat 6. However, the specific details of the powersource `are not limited to the embodiment shown and any conventionalmeans may be used.

The boiler 2 is an enclosed vessel `suitable for containing thesilicon-supplying compound 7. The boiler resides in a constanttemperature bath 8 by means of which the temperature of the compound maybe accurately regulated.

We have empirically determined that the depositionof amorphous siliconis carried out most effectively in the presence of certain other gases:such as hydrogen and helium. These gases, in specified amounts andduring specified periods, are admitted to the boiler, to be mixed withthe silicon compound, land/ or to the reactor. TheyI have a singularlybeneficial effect on the deposition of the amorphous layer although justprecisely why this should be so Iis not yet known. A tank of hydrogengas under pressure is shown at 9. A pressure line 10 leads from the tank9 through a yconventional flow meter 11, which measures the rate of flowin volumetric units of gas per unit time, to a three-way valve 12. Bymeans of the valve '12 the hydrogen flow from the tank may be directedso las to be shut off entirely, admitted only to the boiler through line13, admitted both to the boiler through line 13 `and concurrently to thereactor through line 14, or admitted only to fthe reactor through line14. A second tank 15 containing an inert gas such as helium or argon, isconnected in similiar fashion to both the boiler and the reactor,through a flow meter 16, l-ine 17, valve 18, boiler inlet line 19', andreactor inlet line '14. The lower ends of the boiler inlet lines 13`yand 19 depend below the surface of the liquid silane 7, so that whengas is admitted to the boiler, it will bubble -up through the silane andthereby become dissolved in it. To maintain a steady pressure in theboiler, an outlet line 20 leads after it has passed through the boiler.

.to drive all the :air

reactor, thereby preventing from the boiler to a three-way valve 21 bywhich the boiler gases may be bled off, as desired, through line 22.Through line 23` the boiler outlet line 20 may, by turning the valve 2liappropriately, be opened to the reactor inlet line 14, so that silanevapor may be permitted to enter the reactor from the boiler. So that asteady pressure may be maintained in the reactor, the reactor isprovided with a blow-off line 24.

=It must be noted that the foregoing 'apparatus is in no way specifiedas a limitation on the scope of this invention. It is merely anarrangement representative of the type with which the deposition may becarried out, and is included so that the process itself may be betterunderstood.

rFollowing are examples of the specific process of this invention withthe utilization of different hydrogen-containing silanes:

Example 1 A. quantity of trichlorosilane 7 is introduced into the boiler2, sufficient in amount to submerge lthe lower ends of the boiler inletlines 13 and 19. The constant ternperature bath -8 is adjusted `so as tomaintain the boiler and trichlorosilane in it at a uniform temperatureof approximately C., at which temperature the silane is lin a liquidphase. Hydrogen gas from supply tank 9 is admitted through valve 12first to Ithe boiler, valve 21 being opened so that the gas flowsthrough the reactor By means of the ow meter 11, the rate of fiow ismaintained at 50 cubic feet per hour for lapproximately ten minutes, orfor a period `of time sufficient to outgas the apparatus, that is, fromit, so that the atmosphere will be essentially hydrogen.

At the completion of the outgassing, .the hydrogen ow is shut ofi fromthe boiler and is by-passed into the reactor through valve 12, valve 21being closed to retain the hydrogen atmosphere in the boiler. Thereactor is outgassed with hydrogen at 50 cubic -feet per hour forapproximately one hour 'and ytwenty minutes. Hydrogen fiow Iis shut offat valve 12 and 'the reactor is then outgassed with a fiow of heliumfrom tank 15 at the rate of 25 cubic feet per hour, admitted throughvalve 18, vfor 30 minutes. A condition `of steady-state flow through thereactor is permitted by the reactor outlet line 24, which vents theadmitted gas after its passage through the the interial pressure fromr-ising.

After the total period of outgassing, the boiler outlet valve 21 isopened so that trichlorosilane vapors which have accumulated in theboiler through vaporization of the silane are admitted to the reactorthrough line 14. The tantalum filament 3 is resistively heated to ratemperature of approximately l050 C., the helium flow continuing asbefore. The power to heat the filament is drawn from the generator 4,switch 5 being closed. The rheostat 16 may be calibrated so as todirectly indicate filament temperature, yor -the temperature may bedetermined b-y means of a thermocouple located near the filament or bythe use of an optical pyrometer. In regard to the latter means, it is ofcourse necessary that the reactor have a window for viewing thefilament. At a later stage of the process, during deposition itself, theprovision of a window in the reactor is independently advantageousbecause through it one may observe, the power to the filament andpermitting it to cool from its red-hot condition, the color of thedeposit, and thereby` know whether the deposit is amorphous orcrystalline silicon. The window is preferably of clear fused quartz.

Deposition of amorphous silicon begins at the time the filament isheated. Silane vapors, at a temperature of 0 C., and mixed with helium,fiow into the reactor, and as the silane molecules come into contactwith the hot wire, they are given sufficient energy to cause them todissociate in the manner previously described, the resultant elementalsilicon being deposited on the hot wire.

after switching off At these temperatures, silicon is a solid so thatthe deposit is not vaporized by the heat. The substrate is heatconductive and itself acts as the heat source supplying energy ofdecomposition to subsequent sane molecules, becoming red-hot at theoperating temperature. So long as the current to the filament istemporarily shut off, the amorphous silicon may be recognized throughthe quartz window by its characteristic yellowish, golden or reddishcolor. The reactor walls, being at a much lower temperature because ofthe helium blanket which carries the heat away as it leaves the reactorvia the outlet line 24, do not have sufficient heat energy to cause thesilane to decompose and consequently they are not covered with amorphoussilicon but at the same time are warm enough to prevent the silane vaporfrom condensing on them; the window therefore remains clear andunclouded.

When the filament has been heated to this temperature for approximatelyfifteen minutes, hydrogen iioW is again started, being admitted to thereactor only at a rate of 50 cubic feet per hour. Helium flow continuesat a constant rate, both gases by-passing the boiler to the reactor fortwo minutes. After that period, the lines 13 and 1 9 inletting to theboiler are opened at valves 12 and 18 respectively and valve 21 isopened to connect the boiler outlet to the reactor, both hydrogen andhelium flow being directed to fiow first through the boiler and thenthrough the reactor for a period of ten minutes. Next, helium flow, atthe same rate, is directed through the by-pass valve 18 to the reactor,while hydrogen continues to enter the reactor after passing through theboiler. The boiler temperature is gradually brought up to a temperatureof 27 C. by means of the constant temperature bath over a one hourperiod. Helium flow is then shut off entirely at valve 18 and conditionsare maintained at these levels. When the temperature of the silane vaporis raised to 27 C. in the presence of hydrogen, the deposition ofamorphous silicon on the Wire ceases and the deposition of crystallinesilicon begins.

The process is continued under these conditions until the coating buildsup to the desired thickness; for example, deposition over a period ofabout eleven hours builds up a billet of approximately one-fourth inchwall `thickness, as indicated at 25 in FIGURE l. At the end of thistime, the filament temperature is gradually reduced over a period of onehour, during which time the boiler is cooled to about 0 C., while thehydrogen fiow is maintained through the reactor. After reaching roomtemperature, the crystal bar is removed from the reactor, still on thelament. The filament may be pulled away from the bar merely with manualpull by gripping the bar with the hand and the filament with a pair ofpliers. The bar is now ready to be refined without further treatment.

*' The product of the above-described process is a crystalline siliconbar of size ranging up to 46 inches in length -and of a diameter up to,say, 1/2, displaying, upon refining, excellent electrical properties. Itshould be pointed out that the apparatus may be modified to permit thelocation of a plurality of filaments in the reactor, all obtaining powerfrom the same source if desired. This results in a greater productionbut the process is in all respects otherwise similar.

The above steps are not intended to be limiting procedures but arepresented as the general preferred method of practicing our invention toattain satisfactory results.

It will be seen that the deposition of the amorphous layer in the aboveexample is accomplished by maintaining the silane boiler at a relativelylow temperature together with the presence in the reactor of a mixedhydrogen-helium stream. The crystalline silicon deposit, on the otherhand, is provided by a higher boiler operating temperature, i.e. 27 C.,with a stream of hydrogen alone in the reactor. Argon or any other inertgas might be used rather than helium in any of the places where heliumhas been called for.

Example 2 An example of this invention when tribromosilane, SiHBr3, andargon gas are used in place of trichlorosilane and helium is as follows:The tribromosilane having been introduced into the boiler, the reactorand boiler are outgassed with hydrogen at 30 cubic feet per hour for tenminutes, the boiler being maintained at C. The tribromosilane is a lessvolatile compound than that previously referred to, and boils at atemperature of 109 C. at atmospheric pressure. The hydrogen flow to theboiler is shut off and the reactor alone is outgassed for another hourand twenty minutes. Hydrogen liow is then stopped entirely and thereactor is outgassed with helium for one-half hour at a rate of cubicfeet per hour. After this the filament is heated up to the operatingtemperature of 1050 C., argon flow continuing, for fifteen minutes andthe deposition of amorphous silicon begins. The hydrogen flow is againstarted at 40 cubic feet per hour and is directed entirely into thereactor, along with the argon, for two minutes. The lines 13 and 19 tothe boiler are then opened and the argon and hydrogen are allowed tofiow through the boiler as well as into the reactor. After fifteenminutes, the argon is by-passed directly to the reactor. The boiler isnow gradually brought to an operating temperature of approximately 90 C.by means of the constant temperature bath. After hydrogen has passedthrough the boiler for one hour, the argon iiow is stopped and theduration of the run is maintained at these conditions while thecrystalline silicon deposits on the hot wire. The run is continued fortwelve hours, at the end of which the filament temperature is graduallyreduced over a two hour span, and the boiler temperature is reduced to 0C. After the filament reaches room temperature, the crystal is removedfrom the reactor and stripped intact from the filaments and is thenready for Zone refining.

Having described our invention, we claim:

1. A process for preparing crystalline silicone comprising the steps ofdepositing a substrate of powdery silicon in the form of fine discreteparticles on a filament of a material other than silicon, and thendepositing crystalline silicon over said substrate.

2. A process for preparing crystalline silicon comprising the steps ofdepositing on a filament a substrate of powdery silicon in the form oftine discrete particles produced by the thermal decomposition of asilane, depositing crystalline silicon over said substrate, and thenpulling the crystalline silicon deposit longitudinally from theiilament.

3. A process for producing crystalline silicon in which a layer ofamorphous silicon is deposited as a substrate and crystalline silicon isdeposited over said amorphous substrate asa subsequent step of saidprocess, whereby the removal of the crystalline silicon from thematerial on which it is deposited is facilitated by the intermediacy ofsaid substrate.

4. A process for preparing crystalline silicon in bar form, comprisingthe steps of mixing vaporized trichlorosilane wtih hydrogen and heliumgases at a temperature of approximately `0 C., flowing said mixture ofgases over a tantalum filament heated to a temperature of about l050 C.,whereby said trichlorosilane is caused to thermally decompose producinga deposit of amorphous elemental silicon on said, filament, then flowinga mixture of trichlorosilane and hydrogen at a temperature ofapproximately 27 over said heated filament, thereby depositingcrystalline silicone over said amorphous silicon, and subsequentlystripping said crystalline silicon from said filament by pulling saidfilament longitudinally from said silicon.

5. The method of facilitating the removal of a billet of crystallinesilicon from a filament on which said billet has been deposited bythermal decomposition of a siliconcontaining vapor, said methodcomprising initially depositing a layer of amorphous silicon on saidiilament prior to the deposition of said billet of crystalline siliconwhereby said amorphous silicon permits said billet to be easily removedfrom said filament.

6. The method of facilitating the removal of a billet of crystallinesilicon from a filament upon which said billet is deposited by thedecomposition of a silicon-containing gas at an elevated temperature,said method cornprising initially depositing a layer of amorphoussilicon on said filament prior to the deposition` thereon of crystallinesilicon, the deposition of said amorphous silicon Ybeing effected by thedecomposition of the silicon-containlwhich improvement comprisesinitially adding heat energy to a vaporized silane by said hot wire toeffect deposivtion of amorphous silicon on said hot wire by thermaldecomposition ofV said silane and thenl depositing crystalline siliconover said amorphous silicon.

l1. A method of preparing crystalline silicon comprising the steps ofdepositing a layer of amorhpous silicon onto a body and thereafterdepositing crystalline silicone onto said layer of amorphous silicon.

12. The method comprising, depositing an initial powdery vehicular layerof silicon in the form of fine discrete particles onto a body andthereafter depositing silicon in solid continuous form over said layer.

References Cited in the file of this patent FOREIGN PATENTS GreatBritain Feb. `2,9, 1956 OTHER REFERENCES FIAT Final Report 789,Experiments to Produce Ductile Silicon, April 3, 1946, 5 pages.

Sangster et al.: Journal of the Electrochemical Society (1957), vol.104, No. 5, pp. 317-319.

Szekely: Journal of the Electrochemical Society (1957), V61. 104, No.11, pp. 663-667.

Pfann: Zone Melting, John Wiley-and Sons, Inc., 1958, p. 89.

2. A PROCESS FOR PREPARING CRYSTALLINE SILICON COMPRISING THE STEPS OFDEPOSITING ON A FILAMENT A SUBSTRATE OF POWDERY SILICON IN THE FORM OFFINE DISCRETE PARTICLES PRODUCED BY THE THERMAL DECOMPOSITION OF ASILANE, DEPOSITING CRYSTALLINE SILICON OVER SAID SUBSTRATE, AND THENPULLING THE CRYSTALLINE SILICON DEPOSIT LONGITUDINALLY FROM THEFILAMENT.